Add expand_depth test buttons to Pivot toolbar

Temporary UI for testing programmatic row expansion control via
plugin_config.expand_depth in Perspective viewer.restore().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/pages/Pivot.jsx

@@ -286,6 +286,22 @@ export default function Pivot({ source }) {
         )}
 
         {layoutMsg && <span className="text-xs text-green-600 ml-1">{layoutMsg}</span>}
+
+        {/* expand_depth test buttons */}
+        <div className="ml-auto flex items-center gap-1">
+          <span className="text-xs text-gray-300">expand test:</span>
+          {[0, 1, 2, 3].map(d => (
+            <button key={d} onClick={async () => {
+              const v = viewerRef.current; if (!v) return
+              const cfg = await v.save()
+              const next = { ...cfg, plugin_config: { ...cfg.plugin_config, expand_depth: d } }
+              console.log('trying plugin_config expand_depth:', d, next)
+              await v.restore(next)
+            }} className="text-xs border border-gray-200 rounded px-1.5 py-0.5 text-gray-500 hover:border-gray-400">
+              {d}
+            </button>
+          ))}
+        </div>
       </div>
 
       {/* Pivot + inspector */}